1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and a separator. The membrane electrode assembly includes pair of electrodes and an electrolyte membrane interposed between the electrodes. A reactant gas flow field is formed in the fuel cell for supplying a reactant gas along an electrode surface. An inlet buffer is connected to an inlet of the reactant gas flow field. An outlet buffer is connected to an outlet of the reactant gas flow field.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. In use, normally, a predetermined numbers of power generation cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field (reactant gas flow field) for supplying a fuel gas to the anode, and an oxygen-containing gas flow field (reactant gas flow field) for supplying an oxygen-containing gas to the cathode are formed in the surfaces of the separator. Further, a coolant flow field as a passage of a coolant is formed along the surfaces of the separators for each power generation cell or for every predetermined number of power generation cells.
The fuel cell may adopt internal manifold structure in which reactant gas supply passages and reactant gas discharge passages, a coolant supply passage, and a coolant discharge passage extending through the separators are provided in the fuel cell.
In the fuel cell of this type, in general, an inlet buffer is provided between the reactant gas supply passage and the reactant gas flow field for supplying the reactant gas such that the reactant gas is distributed uniformly to the reactant gas flow field. Further, an outlet buffer is provided between the reactant gas flow field and the reactant gas discharge passage for discharging the reactant gas such that the reactant gas is merged uniformly into the reactant gas discharge passage.
For example, in Japanese Laid-Open Patent Publication 2006-172924, as shown in FIG. 10, a gas flow field 2 of a separator 1 includes a main flow field area 2a, a distribution area 2b, and a merge area 2c. The main flow field area 2a has a flow field width wider than the channel width at the inlet to an inlet manifold 3a and the channel width at the outlet to an outlet manifold 3b, and has ribs 4a dividing the flow field into a plurality of parts. In the distribution area 2b, ribs 4b are provided between the inlet to the inlet manifold 3a and the main flow field area 2a for dividing the flow field into a plurality of parts. In the merge area 2c, ribs 4c are provided between the outlet to the outlet manifold 3b and the main flow field area 2a for dividing the flow field into a plurality of parts. Gaps 5a, 5b for re-distribution and re-merging are provided between the ends of the ribs 4b, 4c and the ribs 4a of the main flow field area 2a. 
At least one of the ribs 4b, 4c dividing the flow field extends from the main flow field area 2a, the distribution area 2b, or the merge area 2c to each of longitudinal positions on the separator 1 where the gaps 5a, 5b for re-distribution and re-merging are present.
In the conventional technique, the distribution area 2b and the merge area 2c have the same structure. Therefore, the distribution area 2b and the merge area 2c have the same flow resistance. In particular, in the case where the gas flow field 2 is a fuel gas flow field for supplying pure hydrogen to the anode, the hydrogen is consumed in the fuel gas flow field by power generation, and the flow rate in the outlet side of the fuel gas flow field becomes small in comparison with the inlet side of the fuel gas flow field.
In the conventional technique, the pressure loss in the distribution area 2b is larger than the pressure loss in the merge area 2c, and the hydrogen is supplied to the portion of the flow field near the inlet manifold 3a preferentially. In the portion of the flow field remote from the inlet manifold 3a, hydrogen shortage occurs. Accordingly, the power generation performance is lowered, and degradation of the electrode may occur.